0.8.1 — derive positionedBlockCount

Fixed

• positionedBlockCount is derived, not stored. The 0.8.0 initializer took the count as
  its own parameter while positionedBlocks defaulted to empty, for source compatibility —
  so a caller using the old signature produced a nonzero count beside an empty array,
  contradicting the equality documented one line above it. The count is now computed from
  the blocks and the initializer requires them, making the invariant structural instead of
  promised. Source-breaking only for code constructing DocumentInspection by hand.
  Reported by external review.